Sour rot, a disease affecting grapes in viticultural regions worldwide has never been clearly defined. Symptoms of the disease include browning of the berry skin, oozing of the berry pulp and the smell of acetic acid, all in the presence of Drosophila spp. We established a method of diagnosing sour rot that includes (i) a rating scale for characterizing visual symptoms of sour rot, which includes the defining characteristic of loss of berry integrity, and (ii) a quantitative measurement of acetic acid content within the berry. Through the isolation of microbes associated with sour rot, and inoculation experiments, we identified several yeast (Metschnikowia spp., Pichia spp., Saccharomyces sp.) and acetic acid bacteria (Acetobacter sp. and Gluconobacter spp.) that successfully cause sour rot symptoms, when in the presence of Drosophila fruit flies. We conducted three years of replicated field trials on the Vitis interspecific hybrid cv. Vignoles, in which we targeted these organisms through pre-harvest applications of various antimicrobial agents and an insecticide both alone and in combination. In a separate set of experiments, the use of Illumina sequencing allowed us to characterize the microbial changes on the grape berry surface at five key phenological stages: pea-sized, bunch closure, Veraison, 15° Brix and harvest in 2014 through 2016 in the Finger Lakes, New York, and 2016 in Tasmania, Australia. The results of this study suggest that terroir is dynamic at the microbial scale, varying significantly not just between regions but also within a region and among years. Finally, grape endophytic microbes were isolated on media conducive to fungi or bacteria and subsequently identified by Illumina sequencing. Species of the yeast genera Metschnikowia, Pichia, and Hanseniaspora were recovered from every set of samples, as were species of the bacterial genera Acinetobacter, Burkholderia and Bacillus; species of the bacterial genera Acetobacter and Gluconobacter also were recovered from vineyard samples from New York and Tasmania and from supermarket-purchased grapes. The endophytic presence of these microbes within grape berries has implications with respect not only to the potential development of sour rot but also to the broader concept of microbial terroir.